1. Field of the Invention
The present invention is directed to a delay line circuit arrangement and an ultrasonic imaging apparatus utilizing the same.
2. Description of the Invention
In general, a conventional ultrasonic imaging apparatus employs a circuit arrangement of an analogue delay line DL that is mainly constructed by inductors and capacitors so as to steer and also focus echo signals derived from a transducer during the signal reception. For ultrasonic pulse transmission, a digital delay line is normally utilized, which is technically different from the analogue delay line.
A typical analogue delay line circuit arrangement is shown in FIG. 1. The delay line DL has a plurality of taps for various delay times, e.g., 0.8 .mu.S. An analogue switch ASW is connected to these selection taps so as to select the desirable delay time. Between an input terminal IN and an output terminal OUT, both the delay line DL and the analogue switch ASW are connected, and matching resistors Rm are also connected.
The circuit arrangement of the delay line DL shown in FIG. 1 is arranged in such a manner that a plurality of delay line elements are connected in a cascade connection, each of which has its own delay time of approximately, e.g., 10 ns (0.01 .mu.S). Accordingly, the desirable delay time can be easily obtained by assembling the given number of cascade-connected delay line elements.
FIG. 2 shows a circuit diagram of the typical delay line element. The delay line element is constructed by inductors L.sub.1, L.sub.2 and capacitors C.sub.1, C.sub.2. These circuit elements practically function as a filter or the like for the received echo signals.
When the analogue delay line circuit having the delay time of 5 .mu.S is designed, a hundred pieces of the delay line elements having, for example, 0.05 .mu.s delay time are connected in a cascade arrangement.
Consequently, the total insertion loss and the entire amplitude decrease in the high frequency range are adversely influenced by multiplying those of a single delay line element by 100. It is therefore very difficult to realize such an analogue delay line circuit having a delay time in the order of several microseconds for higher frequencies, e.g., 10 MHZ. For instance, if the maximum delay time is selected to be 5 .mu.s, the effective maximum frequency is limited to approximately 7 MHZ (-3 dB below).
Recently, there has been a tendency to introduce an ultrasonic imaging apparatus for utilizing high frequency transducers. Accordingly, the above-described conventional transducer is not sufficient for these current imaging apparatus.
The major cause for the RF frequency problems of the delay line circuit, i.e., an abrupt decrease in the high frequency range of the delayed echo signals, is as follows. A magnetic core that can be driven in the higher frequency of, say, 10 MHZ is not available. Precisely speaking, since there exists a size-limitation on the analogue delay line circuit, the effective sizes of the inductors L.sub.1 and L.sub.2 are restricted to given small values. Practically, a toroidal core having a diameter of 10 mm is employed. It is, of course, possible to realize a desired RF frequency characteristic if the toroidal core having a larger diameter than the above core could be utilized.
The other cause for poor performance of the delay line circuit is an insertion loss of the echo signals. As previously described, the longer the desirable delay time is set, the greater the total insertion loss of the echo signals. That is, if the tap Xn of the analogue switch ASW is selected, the insertion loss is maximum (see FIG. 1). For instance, if the delay time is selected to be about 5 .mu.s, the insertion loss amounts to 1 to 2 dB. Accordingly, the amplitude of the delayed echo signal processed in the delay line circuit attenuates in the higher frequency range (see frequency characteristic curve of FIG. 3) due to both the insertion loss and the high frequency response of the delay line circuit.
As easily seen from the characteristic data of FIG. 3, there exist the insertion loss "l" and the amplitude difference "a" due to the degradation of the frequency characteristic, when the tap for the shortest delay time Xl and that for the longest delay time Xn are selected. In other words, the magnitudes of the amplitudes of the output echo signals differ from each other in accordance with the selected delay time, resulting in the poor image quality of the tomographic image obtained by the ultrasonic imaging apparatus employing such a conventional delay line circuit arrangement that is operated under a higher frequency.
Another drawback of the conventional delay line circuit arrangement is the inherent reflection phenomenon thereof. That is to say, although the echo signals input in the delay line circuit via the selected input tap of the analogue switch ASW flow through the given delay line elements and are derived from the output terminal, a part of the input echo signals flows through the remaining delay line elements that were not selected to the output terminal. As a result, a partial echo signal causes the reflection therein. Then, these reflected echo signals are added to the normally-delayed echo signals having time delays. It should be noted that since degrees of the delay time for both delayed echo signals are different from each other, the reflected echo signals are not superimposed with the normally-delayed echo signals. Accordingly, the artifact appears in the resultant tomographic image.
It is therefore an object of the present invention to provide a delay line circuit arrangement constructed of not only such passive elements as coils and capacitors, but also of such active elements as transistors and integrated circuits, these active elements being interposed between the succeeding delay line elements.
Another object of the invention is to provide an ultrasonic imaging apparatus wherein the specific delay line circuit arrangement is employed so as to compensate for the abrupt decrease in the delayed echo signals and the insertion loss thereof, and further to prevent the echo signal reflections in the inoperative delay line elements.